It is not often that you see one woman aged 81 and another who is a size 14 modelling side by side. In fact it never happens. Until yesterday.

To mark the start of London fashion week, a campaigning group called All Walks Beyond the Catwalk asked photographer Rankin to shoot portraits of a group of nine hand-picked, deliberately "diverse" models.

The women were dressed by the likes of Vivienne Westwood, Stella McCartney, Matthew Williamson, Betty Jackson and Alice Temperley. It is the first time so many big-name designers have turned out in support of a new diversity on the catwalk – or, rather, in defiance of what All Walks' Caryn Franklin, a TV presenter, calls "the default setting of pre-pubescent waif". The shoot took place yesterday morning, seconds from the catwalk tents at central London's Somerset House.

Snapped: A Diverse Portrait of British Fashion is the latest salvo from this group, launched last year, which is trying to change the face of fashion. Debra Bourne, a PR guru and co-founder of the group with Franklin, explains: "It's a real shift since last year, when no one had an opinion about body shape. Now you can't move for soundbites about Christina Hendricks." This, they think, is their moment for action.

All Walks asked Westwood – herself 70 next year – to dress 81-year-old Daphne Self, famous for her long, flowing grey hair. Westwood said: "Ideal beauty is not what interests me." She simply likes a woman, she added, "who takes the trouble to look good and show off her best assets". Hussein Chalayan and two other designers fitted models size 14 or over. However, size and age are just part of the campaign: the idea is also to highlight the absence of women from different ethnic backgrounds.

All Walks continues to court controversy, despite claiming that they are not criticising the fashion industry but simply trying to change things from within.

Last year they introduced up-and-coming designer Mark Fast to the voluptuous size-12 model Hayley Morley. Having used Morley in the All Walks show, Fast went on to use her and other "large" models in his main show, causing a scandal when one stylist apparently walked out over the decision.

The celebrity face of All Walks is model Erin O'Connor who, every fashion week, also oversees a special "Models' Sanctuary" at an undisclosed London hotel. (This is a place where models can relax away from the paparazzi and get beauty treatments and nutritional advice.)

Both projects are perhaps a sign the fashion industry wants to be seen to be looking after its own, after years of bad publicity about the cult of emaciation. Most of all, the campaign is the reaction to the sample size used in catwalk shows. This has shrunk beyond all sense in recent years. "I'd like to see it expanded," says Franklin. "Use a standard woman. Not a woman who is six foot and UK size eight. That was never what Cindy Crawford or Linda Evangelista were. They were much curvier."

Franklin is now in talks with more than 60 colleges about training fashion students to cut clothes on something larger than the usual size eight tailor's dummy. Bourne and Franklin, who describe themselves as "just two mums, doing this from the heart", have been invited by Lynne Featherstone, a Liberal Democrat MP and parliamentary under-secretary for equalities, to "take the issue into government".

They are also part of the Lib Dems' Campaign for Body Confidence, headed by MP Jo Swinson. But can they really change anything in fashion itself?

Last spring's shows were crowded with towering, skeletal figures and this week is likely to see plenty of the same. "We're saying all women are beautiful," Franklin counters. "Some are older. Some are standard-size. We're not in conflict with the fashion industry. We're just saying, 'Let's celebrate a broader range'." In all senses of the term.